All U.S. patents cited below are herein entirely incorporated by reference.
Polypropylene has long been known to exist in several forms. Generally, isotactic propylene (iPP) can be described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the polymer chain, whereas syndiotactic polypropylene (sPP) may generally be described as having the methyl groups attached on alternating sides of the polymer chain. More specifically, the isotactic structure is typically described as having the methyl groups attached to the tertiary carbon atoms of successive monomeric units on the same side of a hypothetical plane through the main chain of the polymer, e.g., the methyl groups are all above or all below the plane. A more thorough description of syndiotactic polypropylene is found in col. 1, line 19 of col. 3 of U.S. Pat. No. 5,969,021 to Reddy et al., particularly in comparison with the general configurations of isotactic polypropylene.
Thus, syndiotactic polymers, in contrast to the isotactic structure, are those in which the methyl groups attached to the tertiary carbon atoms of successive monomeric units in the chain lie on alternate sides of the plane of the polymer, thereby creating a configuration of successive methyl groups on alternate sides of a common plane (i.e., racemic dyads). The percentage of racemic dyads in the chain thus determines the degree of syndiotacticity of the polymer. Syndiotactic polymers are crystalline and, like the isotactic polymers, are insoluble in xylene (although the degree of crystallinity of s-PP is much less than for i-PP). This crystallinity distinguishes both syndiotactic and isotactic polymers from an atactic polymer which is amorphous, and soluble in xylene. Atactic polypropylene exhibits no regular order of repeating unit configurations in the polymer chain and forms essentially a waxy product. Furthermore, s-PP exhibits a far lower melting point (˜128° C.) than for i-PP (˜153-170° C.).
Such noticeable physical differences in comparison with i-PP allow for utility of s-PP in specialized applications. For example, syndiotactic polypropylene provides such improved characteristics over isotactic types, namely, but not limited to, clarity, toughness (impact resistance), and feel (softness), as well as surface smoothness and uniformity, some improvements to an extraordinary degree. Thus, end-uses such as dental retainers, medicine droppers, eye droppers, and pen caps normally include s-PP as the primary polymer. However, historic utilization of such a polymer within other fields has been severely limited due to problems associated with low crystallization rates and crystallization temperatures (and thus inordinate cost levels), as well as low flexural modulus characteristics.
As noted within the citation to Reddy et al., above, the crystallization rate of syndiotactic polypropylene is much slower than that for isotactic PP due the low retention rate of crystallinity exhibited by syndiotactic polypropylene in general. In fact, syndiotactic polypropylene continues to crystallize even after pelletization thereof during the production of the resin throughout the process. Furthermore, such low crystallization temperatures also require cooling of injection molded parts or extruded films or sheets to much lower temperatures, and in some cases, higher than needed, for example, for isotactic polypropylene, thereby adding to the cost of manufacture as well due to slower production rates and increased energy costs. Additionally, not only does syndiotactic polypropylene have crystallization temperatures and rates that could be improved, some common processing additives, such as calcium stearate, tend to reduce the crystallization rate even more.
Such slow crystallization rates are generally attributable to the fact that syndiotactic polypropylenes exhibit polymorphism; that is, three different crystal types, namely cell I, cell II, and cell III structures therein. Due to the inherent similarities between Cell I and Cell III, only Cell III will be referred to hereinafter. It has been traditionally noted that the manufacture of syndiotactic polypropylene requires two distinct steps after melting of the target resin in order for the crystal structures to change from one structure to another and finally to the desired final type (cell II). The initial crystal configuration, known as cell II (which includes anti-chiral helices along the “a” crystallographic axis and chiral helices along the “b” axis), appears quickly during cooling and is kinetically favored. The cell III crystal structure (which includes anti-chiral helices along both the “a” and “b” crystallographic axes) then forms at relatively high temperatures (thermodynamically favored) during cooling of the molten resin form. Such a cell III structure for syndiotactic polypropylene (higher melting), although formed more slowly, is always in competition with the Cell II form (lower melting), and is not desirable since it is a rubbery, opaque phase and cannot be pelletized during processing. In order to effectuate proper cell II formation, generally such rubbery cell III-type s-PP must then, in general non-limiting terms, be extruded into strands and wound around a spool to permit further cooling and thus generation of the necessary rigidifying cell II configuration. Thus, it appears that the crystallization rate of syndiotactic polypropylene is dictated by the amount of cell III crystals present and thus requiring generation and arrangement within the target polymer. Without intending on being limited to any scientific theory, it appears that proper and useful syndiotactic polypropylene articles should include higher amounts of the rigid cell II crystal types. The apparent problem in the past with slow crystallization procedures lies in the presence of large amounts of cell III crystals therein. A lower amount of such cell III crystal types would thus permit more thorough crystallization of the overall syndiotactic polypropylene during the initially formed cell II phase, thereby reducing the amount of time required for full crystallization thereof. As a result, it is believed that a syndiotactic polypropylene production method that generates higher amounts of cell II crystal structures than cell III types would exhibit faster crystallization (or at least higher crystallization temperatures) and thus would reduce the cost and complexity associated with producing syndiotactic polypropylene articles. In effect, then, such an improved method would permit more widepread use of such an excellent performing class of polypropylenes. As it is today, the dichotomy in melting forms between cell II and III crystal structures causes a processing problem in terms of complexity and time and thus such syndiotactic polypropylene types have proven very difficult to efficiently produce, even with certain nucleating agents present. Thus, as noted above, the need appears to be to reduce the amount of cell III present therein, thus permitting the cell II component to dictate polymer formation, preferably wherein more cell II form is produced than cell III crystals. Furthermore, it appears that a reduction in the amount of rubbery cell III crystals within target s-PP formulations leads to the formation of more flexurally stable end-product articles as well (e.g., the fewer rigid cell II crystals are present, the polypropylene tends to be more flexible and less susceptible to breaking). Unfortunately, to date no such specific advancement has been proffered to the syndiotactic polypropylene industry wherein the majority of s-PP crystals are Cell II.
Thus, in spite of the advancements in the prior art relating to syndiotactic polypropylene, there is a need for a method of improving the crystallization rate and temperature of syndiotactic polypropylene, in addition to the stiffness of final s-PP articles. As a result, then, there is also a need in the art for improving such characteristics, particularly in order to allow for utility of s-PP within larger market areas, particularly those which are today dominated by isotactic polypropylene. To date, no such advancements have been made available increasing the crystallization rates of syndiotactic polypropylene to levels acceptable to displace isotactic polypropylene as the base polymer in certain end-uses.